Humboldt University of Berlin

About: Humboldt University of Berlin is a education organization based out in Berlin, Germany. It is known for research contribution in the topics: Population & Transplantation. The organization has 33671 authors who have published 61781 publications receiving 1908102 citations. The organization is also known as: Humboldt-Universität zu Berlin & Universitas Humboldtiana Berolinensis.

Social network changes and life events across the life span: a meta-analysis.

Abstract: For researchers and practitioners interested in social relationships, the question remains as to how large social networks typically are, and how their size and composition change across adulthood. On the basis of predictions of socioemotional selectivity theory and social convoy theory, we conducted a meta-analysis on age-related social network changes and the effects of life events on social networks using 277 studies with 177,635 participants from adolescence to old age. Cross-sectional as well as longitudinal studies consistently showed that (a) the global social network increased up until young adulthood and then decreased steadily, (b) both the personal network and the friendship network decreased throughout adulthood, (c) the family network was stable in size from adolescence to old age, and (d) other networks with coworkers or neighbors were important only in specific age ranges. Studies focusing on life events that occur at specific ages, such as transition to parenthood, job entry, or widowhood, demonstrated network changes similar to such age-related network changes. Moderator analyses detected that the type of network assessment affected the reported size of global, personal, and family networks. Period effects on network sizes occurred for personal and friendship networks, which have decreased in size over the last 35 years. Together the findings are consistent with the view that a portion of normative, age-related social network changes are due to normative, age-related life events. We discuss how these patterns of normative social network development inform research in social, evolutionary, cultural, and personality psychology.

Making wald tests work for cointegrated VAR systems

Abstract: Wald tests of restrictions on the coefficients of vector autoregressive (VAR) processes are known to have nonstandard asymptotic properties for 1(1) and cointegrated systems of variables. A simple device is proposed which guarantees that Wald tests have asymptotic X2-distributions under general conditions. If the true generation process is a VAR(p) it is proposed to fit a VAR(p+l) to the data and perform a Wald test on the coefficients of the first p lags only. The power properties of the modified tests are studied both analytically and numerically by means of simple illustrative examples.

Neuroplasticity in old age: sustained fivefold induction of hippocampal neurogenesis by long-term environmental enrichment.

Abstract: Neurons are continually born from endogenous stem cells and added to the dentate gyrus throughout life, but adult hippocampal neurogenesis declines precipitously with age. Short-term exposure to an enriched environment leads to a striking increase in new neurons, along with a substantial improvement in behavioral performance. Could this plastic response be relevant for explaining the beneficial effects of leading "an active life" on brain function and pathology? Adult hippocampal neurogenesis in mice living in an enriched environment from the age of 10 to 20 months was fivefold higher than in controls. Relatively, the increase in neuronal phenotypes was entirely at the expense of newly generated astrocytes. This cellular plasticity occurred in the context of significant improvements of learning parameters, exploratory behavior, and locomotor activity. Enriched living mice also had a reduced lipofuscin load in the dentate gyrus, indicating decreased nonspecific age-dependent degeneration. Therefore, in mice signs of neuronal aging can be diminished by a sustained active and challenging life, even if this stimulation started only at medium age. Activity exerts not only an acute but also a sustained effect on brain plasticity.